An image displayed on a cathode ray tube (CRT) may suffer from imperfections or distortions such as defocusing or nonlinearity that is incident to the scanning of the beam on the CRT. Such imperfections or distortions occur because the distance from the electron gun of the CRT to the faceplate varies markedly as the beam is deflected, for example, in the horizontal direction. Reducing the defocusing that occurs as the beam is deflected in the horizontal direction, for example, may be obtained by developing a dynamic focus voltage having a parabolic voltage component at the horizontal rate and applying the dynamic focus voltage to a focus electrode of the CRT for dynamically varying the focus voltage. It is known to derive the parabolic voltage component at the horizontal rate from an S-correction voltage developed in an S-shaping capacitor of a horizontal deflection output stage.
A television receiver, computer or monitor may have the capability of selectively displaying picture information in the same CRT using a deflection current at different horizontal scan frequencies. When displaying the picture information of a television signal defined according to a broadcasting standard, it may be more economical to utilize a horizontal deflection current at a rate of approximately 16 KHz, referred to as the 1f.sub.H rate. Whereas, when displaying the picture information of a high definition television signal or a display monitor data signal, the rate of the horizontal deflection current may be equal to or greater than 32 KHz. The higher rate is referred to as 2nf.sub.H. The value n is equal to or greater than 1.
In the horizontal deflection circuit output stage of a video display monitor capable of operating at multi-scan rates, it is known to vary the number of in-circuit S-capacitors using switched S-capacitors. The selection of the S-capacitors is made automatically via selectable switches, in accordance with the selected horizontal deflection frequency.
When a non-switched retrace capacitor is employed, the length of the horizontal retrace interval is the same at different horizontal frequencies. As a result, the required amplitudes of the S-correction voltage at the different frequencies may be different. Whereas, the required amplitudes of the parabolic voltage component of the dynamic focus voltage may need be the same. Therefore, it may be desirable to control the amplitudes of the parabolic voltage component of the dynamic focus voltage at the different horizontal frequencies separately from the way the amplitudes of the S-correction voltage are controlled.
In carrying out an inventive feature, a parabolic horizontal rate voltage is developed in the S-shaping capacitor. The parabolic voltage is attenuated through a controlled variable voltage divider. The output of the voltage divider is coupled to an input of a differential amplifier that compares and adjusts the peak-to-peak amplitude of the parabolic voltage to be equal to a voltage reference. Adjusting the peak-to-peak amplitude is done by controlling the resistance of a photo resistor of the controlled variable voltage divider. The attenuated parabolic voltage is amplified in a high voltage amplifier. Thereby, the attenuation varies to control the amplitudes of the parabolic voltage component of the dynamic focus voltage at different horizontal frequencies.
In carrying out another inventive feature, the parabolic voltage is attenuated, instead, through a voltage divider coupled to a switch and the attenuated parabolic voltage is amplified in the high voltage amplifier. The switch of the attenuator responds to a switching control signal that is indicative of the selected horizontal frequency. Thereby, the attenuation varies to control the amplitudes of the parabolic voltage component of the dynamic focus voltage at different horizontal frequencies.
A video imaging apparatus, embodying an inventive feature, includes a source of a first parabolic signal at a frequency related to a deflection frequency, selected from a plurality of frequencies. The first parabolic signal has an amplitude determined in accordance with the selected frequency. A voltage divider including a resistor has an input coupled to the source of the first parabolic signal to generate an attenuated parabolic signal having an amplitude in accordance with a value of the resistor. A control circuit, responsive to a control signal that is indicative of the first parabolic signal amplitude, is coupled to the resistor for varying the value of the resistor, in accordance with the control signal. An amplifier responsive to the attenuated parabolic signal and coupled to the focus electrode amplifies the attenuated parabolic signal to generate a dynamic focus voltage at the focus electrode.